1. Field of the Invention
The present invention relates to a controller for controlling a.c. (alternating current) power in a pulse-width modulation (PWM) fashion in a multiple inverter which transforms d.c. (direct current) power into high-voltage, high-capacity a.c. power with variable frequency and/or variable voltage. Particularly, the invention relates to a PWM controller for a multiple inverter circuit which produces the output voltage and current with synthesized voltage and current waveforms by use of several 3-phase inverter circuits connected through a transformer circuit for coupling inductively two electrical circuits, wherein the controller employs the PWM method in which several pulse trains are produced in a half cycle of the a.c. output waveform and the voltage in proportion to the pulse width is varied in a sinusoidal fashion so that a smooth a.c. output is produced and semiconductor switches in inverter main circuits are controlled for their on-off switching operations at the rising and falling edges of pulse signals formed in the pulse trains.
2. Description of the Prior Art
Recently, inverters using semiconductor switching devices such as transistors and thyristors are being used widely for the variable-speed drive control of a.c. motors such as squirrel-cage induction motors for example. At the same time, the recent advances in power electronics technology has enabled these inverters to deal with higher voltages and increased power capacities and made them applicable to power converting facilities with variable frequency and/or variable voltage capabilities.
Various novel techniques have been proposed recently also for controllers which operate in on-off mode semiconductor switches in the inverters. A most advanced controller for driving the gates of semiconductor switching devices is designed to produce multiple pulse trains within a half cycle of the output waveform, with each pulse having a pulse width varied so that the equivalent voltage expected of the pulse train has a sinusoidal waveform, and the gates of switching devices are controlled in PWM mode.
An example of the above-mentioned PWM controller intended for 12-phase PWM inverter multipled by multi-phase transformer is disclosed in the technical report (part II), No. 162, p. 52, entitled "Technical Trend of Self Turn-Off Devices Application in Power Converting Facilities", Institute of Electrical Engineers of Japan, published in January 1984. The arrangement of this PWM inverter circuit is illustrated in FIG. 1. The inverter circuit consists of a smoothing capacitor 1 for smoothing voltage supplied from a d.c. power source, 3-phase inverter main circuits 2 and 3 in parallel connection for converting the d.c. power into a.c. power, PWM pulse signal generators 4 and 5 for producing on-off pulse signals 6 and 7 which separately turn on or off switching devices (not shown) included in the 3-phase inverter main circuits, and a multiple transformer 8 which combines output voltages V.sub.a and V.sub.b of the inverter main circuits 2 and 3 to produce an output voltage V.
The operation of the above PWM inverter circuit will be described. An output voltage command V* and output frequency command f* are fed to the PWM pulse signal generators 4 and 5, which then produce on-off pulse signals 6 and 7, respectively, so that the fundamental waves of the output voltages V.sub.a and V.sub.b of the 3-phase inverter main circuits 2 and 3 have a 30.degree. phase difference. These output voltages V.sub.a and V.sub.b are derived from the inverter input d.c. voltage V.sub.dc, with their waveforms being pulse-width modulated. Their fundamental waves having a phase difference of 30.degree. from each other are combined by the multiple transformer 8, resulting in the output voltage V. The output voltages V.sub.a and V.sub.b of the 3-phase inverter main circuits 2 and 3 have their fundamental waves V.sub.af and V.sub.bf summed by vectorial composition to become an output voltage fundamental V.sub.f, as shown in the vectorial diagram of FIG. 2. The multiple transformer 8 is designed to have winding ratios so that the fifth and seventh harmonic voltage components are cancelled.
In the conventional PWM controller for the multiple inverter arranged as described above, individual inverters have independent switching operations, causing possibly the transformer output voltage to vary in steps from the peak voltage to the zero voltage when both inverters produce zero-voltage vectors simultaneously, as shown in FIG. 1. On this account, when this multiple inverter is used in applications dealing with a high voltage and large power capacity, the voltage step has an increased rate of change on the time axis (i.e., dv/dt), the load current includes increased harmonic ripple components, and a voltage surge is created in some cases, resulting in an adverse influence on devices and facilities at the load side.
The conventional PWM controller needs to have PWM pulse signal generators in which the phase difference of output fundamental waves is set to 30.degree. in the case of 12-phase configuration, individually for each inverter, resulting in a complex and expensive overall system including the PWM controller and inverter main circuits.
Moreover, the inverter operating for a high output voltage causes a large variation in the output voltage under PWM control, creating a high voltage surge applied to the load, and creates an increased current ripple, making difficult the setting of a lower switching frequency for the PWM control for the switching devices in each inverter.